Heart, like many legacy bands in the 1980s, struggled to marry its original sound with the synthesized style of the day. Strange Euphoria, a sweeping new compilation due June 5, 2012 from Epic-Legacy, charts that journey, as Heart transforms itself from hippie-chick singer-songwriters, to feminist heavy rockers, to glossy MTV stars, and back again.

Amongst this 3-CD, 1-DVD collection’s impressive list of previously unreleased tracks is a deeply innocent 1975 folk demo of “Magic Man,” eventually Heart’s initial Top 10 single: Ann Wilson sounds more like a choir girl than the man-breaking hellion who eventually sent this track hurtling out of car speakers everywhere.

Meanwhile, the demo for “Crazy On You,” also previously unreleased from the same session, is a showcase for Nancy’s hopped-up acoustic style — which, with the addition of Roger Fisher’s explosively inventive work on the electric guitar, would define the early Heart sound. (Nancy’s interests ran further afield than just rock, of course; she even tried her hand at classical stylings on “Silver Wheels II,” included here from 1980’s Bebe le Strange.)

That delicate side would eventually become one half of what made Heart whole, something that plays out on Strange Euphoria as pastoral tracks like “Sylvan Song,” included here from 1977’s Little Queen, the acoustic demo of “Dog and Butterfly” and the set-closing “Avalon” from 1993 are paired with claw-your-eyes-out rock tunes like 1977’s “Kick It Out” and 1980’s feminist anthem “Even It Up.” Some of the best of their music, as with “Little Queen” and “Queen City” from 2010’s Red Velvet Car, manages somehow to combine all of that into one. That might be best heard on an absorbing new edit of 1976’s “Dreamboat Annie,” which like “Crazy on You” was a Top 40 hit. Here, several takes are sewn together, moving from a plucky Americana feel to these sweeping orchestral washes and then back to its dreamscape conclusion — like a synopsis of Heart’s early ambitions.

Much of that period’s furiously imaginative verve is captured on the DVD, which features Heart performing in early 1976 on the KWSU program “The Second Ending.”

Strange Euphoria begins with “Through Eyes and Glass,” the initial side made by the duo, recording in 1969 as Ann Wilson and the Daybreaks. (They were so little known then that Nancy’s name was left off the credits.) As the sisters’ songwriting matures, you hear the impact of the hit music surrounding them.

Check out the early sound-check run through of “Heartless,” where Ann’s vocal style bears more than a passing resemblance to Elton John’s. (Ironically, Bernie Taupin — Elton’s longtime writing partner — helped compose the Nancy-sung “These Dreams,” Heart’s 1984 charttopper.) Even into the 1980s, you can hear the siblings’ youthful passion for the music of Aretha Franklin, the Chiffons and Ann Peebles on the previously unreleased demo of a track called “Unconditional Love.” Then, of course, there’s the Beatles — a love that sees if most perfect, if devastating flowering in the 1982 track “Angels,” which frames their emotions in the aftermath of John Lennon’s murder.

That said, no single influence seems to hold sway more than Led Zeppelin, though even that one could never completely define this often indefinably inventive group.

Heart’s live take on “Barracuda,” from a 1977 show at the Universal Amphitheatre in Los Angeles, is so rambunctious that it threatens to rattle everyone right off the stage. With its galloping rhythms and slashing guitar, that track was perhaps the earliest, most overt reference Zeppelin’s impact on Heart, but certainly not the last: 1977’s “Love Alive,” so full of the atmospherics from “The Battle of Evermore,” shows how they would build upon that early influence. (Order through Amazon, and you’ll also get five of their famous Led Zeppelin covers on an extra disc: “Going to California,” “Battle of Evermore,” “What is and What Should Never Be,” “Immigrant Song” and “Misty Mountain Hop” on the Heart Zeppish CD.) Later in the boxed set, Zeppelin’s John Paul Jones sits in for a completely rebuilt live version of “Never,” the 1984 No. 4 hit, adding string charts and a delectable mandolin in a deeply revealing performance from Seattle held a decade later.

Of course, by then, Heart had begun to emerge from a period in the 1980s in which the band found its greatest chart successes, but did so with a pre-packaged image and often with other people’s songs. They gained new fans, while losing many of their old ones. Strange Euphoria highlights, for those who may have missed it, how Heart has rebuilt its own creative momentum, from Nancy Wilson’s 1999 live take on “Everything” (the first song she had written alone, she says, in a decade) to the band’s celebrated Red Velvet Car — an album that cemented the Wilsons’ comeback.

They returned, in many ways, as interesting as they were before — though, alas, they never did find a way to replace Fisher’s unique voicings. Tributes to those serving overseas range from 2004’s rollicking “Fallen Ones,” to the heart-rending 2006 live cut “Lost Angel.” Heart can cook up a scalding city blues on “Love or Madness,” and a nasty bump-and-grind “Skin to Skin,” both previously unreleased. In between, there remains the sweet reverie of hearing the Wilson sisters singing in close harmony, as on “Friend Meets Friend” — another in their long line of songs about the enduring emotions that surround relationships.

Heart, like the title of this endlessly fascinating new box set, still boasts an abiding, very involving complexity. It’s good to be reminded.

Nick DeRiso has explored music for USA Today, All About Jazz and a host of others. Honored as columnist of the year five times by the Associated Press, Louisiana Press Association and Louisiana Sports Writers Association, he oversaw a daily section named Top 10 in the nation by the AP before co-founding Something Else! Nick is now associate editor of Ultimate Classic Rock.